The ability of a solid substance to adsorb water is a function of several factors. Water adsorption occurs when a solid surface is exposed to water or water vapor. The adsorption process is divided into two categories depending on the forces that are involved in the interaction. With physical sorption (i.e., physisorption), water molecules are kept on the surface by weak Van der Waals forces, whereas with chemical adsorption (i.e., chemisorption), water molecules become part of the solid and involve chemical bonding of the water vapor with the solid surface. Thus, physisorption is typically a reversible process and the amount of fluid adsorbed on a surface at equilibrium conditions is dependent on the fluid-solid interactions and the external parameters such as temperature, relative humidity (RH), and vapor pressure. Water molecules adsorb onto the surface of dry solids, forming a monomolecular layer, and as more molecules adhere to the surface, moisture starts transferring into the material via diffusional forces.
Therefore, the total amount of water adsorbed by a solid is usually a sum of the fractions held by different mechanisms. These mechanisms include chemical bonding with the solid; multilayer adsorption; pore or capillarity condensation, responsible for creation of curved interfaces (menisci); and micropore filling. Usually, adsorbed water does not affect a solid until water vapor starts to condensate on the surface at high humidity. Water condensation can trigger a deliquescent process for water-soluble precipitates or components thereof, forming a saturated solution.
The ability of solids to adsorb water vapor can be measured by the sorption or desorption as a function of relative humidity (RH) under constant temperature at equilibrium conditions where sorption or desorption is occurring independent of time. RH is defined through the partial pressure of water vapor as follows:RH=pw/pw0 
Where pw is the partial pressure of water vapor over an aqueous solution and pw0 is the partial pressure of water vapor over pure water. The ratio pw/pw0 is referred to as the relative pressure. The activity of water (aw) in aqueous solutions is related to fugacity by the equation:aw=fw/fw0 
Where fw is the fugacity of water vapor over an aqueous solution and fw0 is the fugacity of water vapor over pure water. At ambient temperature and moderate pressure, water vapor is assumed to behave ideally and the fugacities can be replaced by partial pressure:fw/fwo=pw/pw0 
Yielding:RH=aw where RH is commonly expressed as a percentage; thus RH %=100*aw.
At equilibrium conditions, the water activity on a solid surface is equal to the RH of the atmosphere around the solid. One would expect RH to be the same for sorption or desorption measurements. However, one characteristic of the physisorption process is the hysteresis loop associated with capillary condensation. The hysteresis loop is related to the number and size of the pores on the surface of a solid. A hysteresis loop can be irreversible and the amount of fluid adsorbed does not necessarily coincide with increasing or decreasing vapor pressure over a certain interval.
Solids that adsorb water vapor are often used as desiccants in food, pharmaceuticals, electronics, clothing, and other moisture-sensitive products. Solids that have a high adsorption to mass ratio are preferred, as they can require less space and volume to provide the desired desiccant properties.
Currently, dry silica gel is the most commonly used desiccant. Silica-gel produced from sodium silicate is a porous solid. It consists of porous particles with diameters varying between 2-20 nm that account for a surface area of about 2.8×107 m2 per m3 of silica gel. Due to these properties, the silica-gel can adsorb water vapor. Desiccants with better moisture absorbing capacity than silica-gel are desirable.